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Optical Waves in Anisotropic Crystals

12
Optical Waves in Anisotropic Crystals

An optical crystal is an anisotopic dielectric and the dielectric tensor of such a crystal in a coordinate system that coincides with the principal axes of the crystal will have zero off-diagonal elements and is given by

(12.1)ˉε=[ε1000ε2000ε3].
ε¯=ε1000ε2000ε3.

The principal refractive indices of the crystal are given by

(12.2)nj=(εjε0)1/2,j=1,2,3.
nj=(εjε0)1/2,j=1,2,3.

The crystals are classified as biaxial, uniaxial, or isotropic depending on the number of equalities involving the principal refractive indices:

(12.3)n1n2n3(biaxial crystal),
n1n2n3(biaxial crystal),
(12.4)n1=n2=no,n3=ne(uniaxial crystal),
n1=n2=no,n3=ne(uniaxial crystal),
(12.5)n1=n2=n3=no(isotropic crystal).
n1=n2=n3=no(isotropic crystal).

Properties of wave propagation in isotropic crystals are discussed in Chapter 2. In the following section, we discuss wave propagation in crystals in the order of increasing complexity of analysis [1,2,3].

12.1Wave Propagation in a Biaxial Crystal along the Principal Axes

Let the principal axes be aligned along the Cartesian coordinate system. A uniform plane wave with a harmonic variation in space and time given by

(12.6)f(r,t)=exp[j(ωtkr)]
f(r,t)=exp[j(ωtkr)]

satisfies following equations:

(12.7)k×E=ωB,
k×E=ωB,
(12.8)k×H=ωD,
k×H=ωD,

(12.9)kB=0,
kB=0,
(12.10)kD=0,
kD=0,
(12.11)B=μ0H,
B=μ0H,
(12.12)Dx=ε0n2xEx,Dy=ε0n2yEy,Dz=ε0n2zEz.
Dx=ε0n2xEx,Dy=ε0n2yEy,Dz=ε0n2zEz.

Equation 12.7, Equation 12.8, Equation 12.9, Equation 12.10, Equation 12.11 and Equation 12.12 are obtained from Maxwell’s equations and the constitutive relation for the crystal. From Equations 12.9 and 12.10, we obtain

(12.13)Bk=Dk=0
Bk=Dk=0

and from Equations 12.7 and 12.8, we obtain

(12.14)k×k×E=ω(k×B)=μ0ω2D.
k×k×E=ω(k×B)=μ0ω2D.

For the particular case of

(12.15)k=ˆzk,
k=zˆk,
(12.16)E=ˆxE,
E=xˆE,
(12.17)H=ˆyH,
H=yˆH,

we obtain

(12.18)(k2+μ0ε0ω2n2x)E=0.
(k2+μ0ε0ω2n2x)E=0.

Therefore, the dispersion relation in this case is given by

(12.19)k2=n2xω2μ0ε0.
k2=n2xω2μ0ε0.

The results obtained in this particular case may be stated in words as follows: the propagation of a wave linearly polarized in the direction of a principal axis and propagating along the direction of another principal axis is similar to that of wave propagation in an isotropic case except that the effective refractive index is given by the refractive index in the direction of polarization.

We next consider an example of change of wave polarization by a biaxial crystal. Let the electric field E of a wave propagating in the z-direction in this crystal, at z = 0, be given by

(12.20)E(0)=E0[ˆxcos45°+ˆysin45°],z=0.
E(0)=E0[xˆcos45°+yˆsin45°],z=0.

This wave is linearly polarized at 45° (with the x-axis) in the z = 0 plane. If we consider this wave as a superposition of two waves, one with linear polarization in the x-direction and another with linear polarization in the y-direction, then we can write the electric field of this wave at z = d as

(12.21)E(d)=ˆxE0cos45°exp[j(ωtnxk0d)]+ˆyE0sin45°exp[j(ωtnyk0d)].
E(d)=xˆE0cos45°exp[j(ωtnxk0d)]+yˆE0sin45°exp[j(ωtnyk0d)].

Note that the phase of the wave changes with d since nxk0dnyk0d. Hence, the wave polarization will not remain linear.

12.2Propagation in an Arbitrary Direction

The notation and the formulations in Section 12.2, Section 12.3 and Section 12.4 closely follow that of reference [3]. Let the unit vector in the direction of propagation be designated by ˆe3eˆ3 (see Figure 12.1):

(12.22)k=ˆe3k.
k=eˆ3k.
image

FIGURE 12.1
Orthogonal coordinate system with one coordinate along the wave normal.

Let ˆe1eˆ1 be a unit vector in the xy-plane. The direction of ˆe1eˆ1 is obtained by rotating the projection of ˆe3eˆ3 in the xy-plane by 90° as shown in the figure. It is obvious that

(12.23)ˆe3ˆe1=0
eˆ3eˆ1=0

and

(12.24)ˆe1=(ˆe1ˆx)ˆx+(ˆe1ˆy)ˆy=cos(90ϕ)ˆx+cos(180ϕ)ˆy=ˆxsinϕˆycosϕ.
eˆ1=(eˆ1xˆ)xˆ+(eˆ1yˆ)yˆ=cos(90ϕ)xˆ+cos(180ϕ)yˆ=xˆsinϕyˆcosϕ.

Let

(12.25)ˆe2=ˆe3׈e1=ˆxcosθcosϕ+ˆycosθsinϕˆzsinθ.
eˆ2=eˆ3×eˆ1=xˆcosθcosϕ+yˆcosθsinϕzˆsinθ.

The unit vectors ˆe1,ˆe2,andˆe3eˆ1,eˆ2,andeˆ3 are mutually orthogonal. A vector A may be resolved along xyz coordinates as well as along the directions of ˆe1,ˆe2,andˆe3eˆ1,eˆ2,andeˆ3. In the latter case, let us designate the vector by Ak with components A1, A2, and A3. The transformation that relates the components of A with components of Ak may be written as

(12.26)Ak=ˉTA,
Ak=T¯¯¯A,
(12.27)A=ˉT1Ak.
A=T¯¯¯1Ak.

The transformation is easily determined by noting

(12.28)A1=ˆe1Ak=ˆe1ˆxAx+ˆe1ˆyAy+ˆe1ˆzAz=AxsinϕAycosϕ.
A1=eˆ1Ak=eˆ1xˆAx+eˆ1yˆAy+eˆ1zˆAz=AxsinϕAycosϕ.

Similar considerations will lead to the determination of the transformation matrix ˉTT¯¯¯:

(12.29)ˉT=[sinϕcosϕ0cosθcosϕcosθsinϕsinθsinθcosϕsinθsinϕcosθ].
T¯¯¯=sinϕcosθcosϕsinθcosϕcosϕcosθsinϕsinθsinϕ0sinθcosθ.

By calculating ˉT1T¯¯¯1, we can show that

(12.30)ˉT1=(ˉT)T,
T¯¯¯1=(T¯¯¯)T,

where the superscript T stands for the transpose operation. Defining an impermittivity tensor ˉηη¯ (here ˉηη¯ is not characteristic impedance),

(12.31)η=ε1p,
η=ε1p,

where

(12.32)ˉε=ε0ˉεp,
ε¯=ε0ε¯p,

we obtain

(12.33)E=1ε0ˉηD,
E=1ε0η¯D,
(12.34)ˉT1Ek=1ε0ˉηˉT1Dk,
T¯¯¯1Ek=1ε0η¯T¯¯¯1Dk,
(12.35)Ek=1ε0[ˉTˉηˉT1]ˉDk,
Ek=1ε0[T¯¯¯η¯T¯¯¯1]D¯¯¯k,
(12.36)ˉEk=1ε0ˉηkˉDk.
E¯¯k=1ε0η¯kD¯¯¯k.

If the elements of ˉηkη¯k are designated by ηij, then Equation 12.33 may be written as

(12.37)[E1E2E3]=1ε0[η11η12η13η21η22η23η31η32η33][D1D20].
E1E2E3=1ε0η11η21η31η12η22η32η13η23η33D1D20.

From Equations 12.7, 12.8, and 12.37, we obtain

(12.38)ωB2=kE1=kε0[η11D1+η12D2],
ωB2=kE1=kε0[η11D1+η12D2],
(12.39)ωB1=kE2=kε0[η21D1+η22D2],
ωB1=kE2=kε0[η21D1+η22D2],
(12.40)ωD2=kH1=kB1μ0,
ωD2=kH1=kB1μ0,
(12.41)ωD1=kH2=kB2μ0.
ωD1=kH2=kB2μ0.

By eliminating all other variables, we can obtain the dispersion relation

(12.42)[η11u2c2η12η21η22u2c2][D1D2]=0,
η11u2c2η21η12η22u2c2[D1D2]=0,

where u = ω/k is the phase velocity.

12.3Propagation in an Arbitrary Direction: Uniaxial Crystal

As an example of further calculations, we restrict ourselves to a uniaxial crystal. For this case,

(12.43)ˉηk=[η000ηcos2θ+ηzsin2θ(ηηz)sinθcosθ0(ηηz)sinθcosθηsinθ+ηzcosθ],
η¯k=η000ηcos2θ+ηzsin2θ(ηηz)sinθcosθ0(ηηz)sinθcosθηsinθ+ηzcosθ,

where

(12.44)η=1n2o,
η=1n2o,
(12.45)ηz=1n2e.
ηz=1n2e.

Thus,

(12.46)η11=η,
η11=η,
(12.47)η12=η21=0,
η12=η21=0,
(12.48)η22=ηcos2θ+ηzsin2θ.
η22=ηcos2θ+ηzsin2θ.

From Equations 12.42 and Equation 12.46, Equation 12.47 and Equation 12.48, we obtain the following results.

Case 1:

(12.49)(ηu2c2)=0(D10,D2=0).
(ηu2c2)=0(D10,D2=0).

Case 2:

(12.50)η22=ηcos2θ+ηzsin2θ=u2c2(D1=0,D20).
η22=ηcos2θ+ηzsin2θ=u2c2(D1=0,D20).

In the first case, the phase velocity is

(12.51)u=±cno(ordinary wave)
u=±cno(ordinary wave)

and in the second case it is

(12.52)uc=1n=cos2θn2o+sin2θn2e(extraordinary wave).
uc=1n=cos2θn2o+sin2θn2e(extraordinary wave).

In the latter case, the phase velocity depends on θ.

12.4k-Surface

Let k be the scalar wave number and is given by

(12.53)kz=kcosθ,
kz=kcosθ,
(12.54)ks=ksinθ,

where ks is the transverse wave number.

For Case 1,

(12.55)u=cno=ωk,
(12.56)k2z+k2s=k2=ω2n2oc2.

Figure 12.2a shows this circle in kskz plain.

image

FIGURE 12.2
k-Surface. (a) Ordinary and (b) ordinary and extraordinary.

For Case 2,

(12.57)u2c2=ω2k2c2=cos2θn2o+sin2θn2e,

which may be written as

(12.58)k2sn2e(ω2/c2)+k2zn2o(ω2/c2)=1.

Thus, the curve in kskz plane is an ellipse as shown in Figure 12.2.

The phase velocity is given by ω/k and for the ordinary wave it is the same for all θ. For the extraordinary wave, ω/k depends on θ, since the k curve is an ellipse. The group velocity for this case can be defined as a vector:

(12.59)ug=kω=ˆksωks+ˆzωkz.

In three-dimensional k-space, we obtain a sphere for the ordinary wave and ellipsoid for the extraordinary wave.

For a uniaxial crystal, the refractive index is the same along the two principal directions (no in x- and y-directions in our notation) and different along the axis (ne along the z-axis) called the optical axis. Note that the refractive index surfaces touch along the kz-axis.

12.5Group Velocity as a Function of Polar Angle

The angle between the optical axis and the wave normal (direction of phase propagation) is the polar angle θ. From Equation 12.58, we obtain

ω2=c2[k2zn2o+k2sn2e],

ω=c[k2zn2o+k2sn2e]1/2,

where c is the velocity of light in free space, kz = k cos θ, and ks = k sin θ:

(12.60)ωkz=c2[k2zn2o+k2sn2e]1/2[2kzn2o],
(12.61)ωks=c2[k2zn2o+k2sn2e]1/2[2ksn2e].

Noting that

[k2zn2o+k2sn2e]1/2=(ω2c2)1/2=cω,
ωkz=c22kzn2ocω=kzc2ωn2o=c2kcosθωn2o.

Similarly,

ωks=c2ksinθωn2e,
(12.62)ug=c2kω[sinθn2eˆas+cosθn2oˆaz].

The magnitude of the group velocity |ug| is given by

(12.63)|ug|=c2kω[sin2θn4e+cos2θn4o]1/2.

Since ω/k = u,

(12.64)|ug|u=c2[sin2θn4e+cos2θn4o]1/2.

If the angle of ug with the polar axis is θg, then

tanθg=ugsugz=sinθ/n2ecosθ/n2o=tanθn2on2e.

If α = θ − θg, then

(12.65)tanα=tan(θθg)=tanθtanθg1+tanθtanθg=(1n2o/n2e)tanθ1+(n2otan2θ)/n2e.

Note that if θ = 0, then tan α = 0, and α = 0.

If θ = π/2, then tan α = ∞/∞2 = 1/∞ = 0, thus α = 0 (Figure 12.3).

image

FIGURE 12.3
Various angles for the extraordinary wave.

To find the value of θ so that α is maximum, let tan θ = x, and we have to find x that maximizes x/(1+(n2ox2)/n2e). Differentiating with respect to x and equating to zero (use νduudν = 0), we obtain

[1+n2on2ex2]x[n2on2e2x]=0,
1n2on2ex2=0,
(12.66)x=neno.

α is maximum when tan θ = ne/no:

(12.67)tan(αmax)=ne/no(1n2o/n2e)1+(n2o/n2e)(n2e/n2o)=ne/nono/ne2=n2en2o2none=(ne+no)(neno)2none,tan(αmax)=(ne+no)(neno)2none.

For example, for quartz,

tan(αmax)=(1.553+1.544)(1.5531.544)2(1.553)(1.544)=5.812×103,
αmax=0.333°.

A further approximation can be made for the case none.

For none, tan θ ≈ 1 for α to be maximum.

Hence, θ ≈ 45° for α to be maximum.

From Equation 12.67, we obtain

tan(αmax)2ne(neno)2none=nenono=Δnn=0.0091.544=5.829×103.

αmax = 0.334° nearest to the previous answer.

tan (αmax) is proportional to |none|.

12.6Reflection by an Anisotropic Half-Space

When the transmission medium is anisotropic, we have seen that there are two transmitted waves (Figure 12.4). For oblique incidence, each transmitted wave has its own transmission angle. Since the boundary condition is still that kx is conserved for all waves, Snell’s law may be written as

(12.68)kx=kosinθi=k1sinθT1=k2sinθT2,

where k1 and k2 are the wave numbers of the respective transmitted waves. We have noted that for the extraordinary wave, the k-surface is not a sphere. In such a case, the value of k depends on the angle of refraction. A graphical interpretation of the technique of obtaining the solution is shown in Figure 12.5.

image

FIGURE 12.4
Reflection and transmission by an anisotropic medium.

image

FIGURE 12.5
Computation of the angle of transmission.

References

  1. 1.Saleh, B.E. A. and Teich, M. C., Fundamentals of Photonics, Wiley, New York, 1991.
  2. 2.Yariv, A. and Yeh, P., Optical Waves in Crystals, Wiley, New York, 1984.
  3. 3.Kong, J.U., Electromagnetic Wave Theory, Wiley, New York, 1990.
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